Security software company McAfee have reported the detection of the most widespread infection in 3 years of "Trojan Horse" programs, which install themselves into computers and can be used to steal personal information.
"More than 500,000 infected MP3 music and video files have been found since May 2," said a statement from the company, which is one of the world's largets anti-virus software companies.
The files are mainly spread via Internet "File Sharing" services, or Peer-to-Peer networks.
McAfee have partnered with Yahoo to develop "Site Advisor" software, which will alert McAfee users to sites which have been identified as spreading malicious code. Offending websites will be excluded from Yahoo's search results.